I see no issue. Worst case scenario is that you look up a transfer desk at SIN to get the BP for the SIN-IST-CPT legs. You don't need to enter Singapore to do that as you arrivals and departing pax are not segregated as I recall it, so there is plenty of time.
Even if you have checked luggage ground staff at SIN are known to be prepared to pickup your luggage from the belts and tag it do your new destination whilst you remain in transit; at least this is how it used to work,